Because truck brakes are designed to have the capacity of stopping a vehicle loaded to 20,000 lbs. per axle, they are extremely over-braked while operating empty on slippery roads. Even with ABS operation under these conditions small changes in brake pressure can have a large effect on wheel deceleration and may cause the wheels to be over-braked.
The wheel speed shown in FIG. 1 indicates the effect of this over-braking. Wheel speed control to the reference speed is very poor, producing vehicle stability problems and long stopping distances.
The Y-axis is a multiple scale axis which represents speed in MPH, acceleration in units of g's (.times.10), and numeric value of the ABS logic state. As the logic state on the Y-axis increases, the brake pressure rise rate increases. The X-axis is real time shown in milliseconds, usually ranging from 0 to 6. The acquisition of data is automatically started by the occurrence of a wheel deceleration rate which exceeds the allowable limit, based on vehicle reference speed. The plot contains 0.25 seconds of data prior to the start of the ABS event.
There are four types of data plotted: wheel speed, wheel acceleration, vehicle reference speed and the ABS logic state. The wheel speed data can be identified by its typical cyclic behavior modulating between the actual vehicle speed and the vehicle reference speed. Wheel accelerations correspond to the changes in wheel speed. The reference speed is calculated by subtracting 4 mph from the highest wheel speed and then taking 80% of the result. The reference speed is then decreased at a rate of -0.8 g's until it is recalculated by a higher wheel speed, when data is received. The behavior of the reference speed is typically much less cyclic than the wheel speeds as noted in the plot. The logic states are plotted using triangular markers placed at the Y-axis position corresponding to the numeric value logic state.
Under ideal conditions the wheel speeds should be controlled to the reference speed. When the wheel speeds drop below the reference speed, vehicle stability is decreased. On the other hand, if the wheels remain above the reference speed, the stopping distance may increase. It is important to maintain an accurate reference speed so the proper decisions can be made in controlling the wheel speeds. This becomes a tradeoff of stopping distance because the wheel is free-rolling in order to establish the true vehicle speed.
The root of the control problem is typically in the control logic. Typical control logic can be found in the McNisch, Jr., U.S. Pat. No. 5,071,200. The data of FIG. 1 indicates that the ABS logic recognizes that the wheel is decelerating and signals the ABS valve to release brake pressure. Because of the response time of the brake system, the wheel is over-braked During the Release state, brake torque or pressure is reduced until the wheel starts to accelerate. The control logic then calls for a Hold state on brake pressure until the wheel speed reaches the vehicle reference speed. The wheel acceleration, at the time the wheel pressure exceeds the reference speed, determines how fast brake pressure will be reapplied.
The data further indicates high acceleration levels, up to 6 G's, which, according to the logic, calls for fast pressure application rates. The wheel is again over-braked and the process is repeated, creating an unstable system.
On a high Mu surface this would have been the proper logic for optimum stability and minimum stopping distances, but it does not work here. There are many variables which affect the wheel acceleration and, therefore, it can be misleading when used as the determining factor for brake pressure application rates.
This is a relatively complex problem because it has to be resolved without affecting the performance of the system on high Mu surfaces.